Breast development and tumorigenesis are influenced by hormonal and growth factor signals whose responses are mediated by intracellular and cell membrane receptors. The involvement of intracellular receptors for estrogen and progesterone in the regulation of these processes is well documented. The function of estrogen and progesterone receptors in mammary tissue is influenced by the composition of the receptors themselves and by the factors (hormones, etc.) that regulate their transcriptional regulatory activity. For example, post-transcriptional variants of estrogen and progesterone receptors have been identified that display altered functional activity relative to wild-type receptors. The expression of some of these variants is increased in breast cancer cells. The activity of these variants clearly influences the overall activity of estrogen and progesterone receptors. Secondly, we and others have shown that the activity of steroid receptors for estrogen and progesterone, can be activated in the absence of a s specific steroidal ligand by extracellular signals including growth factors that stimulate intracellular phosphorylation pathways. Our results suggest that these receptors may provide common mediators through which hormone and non-hormone signaling pathways converge to regulate the expression of tar get genes that influence mammary cellular phenotype. The overall objective of this proposal is to examine the mechanistic contribution of estrogen and progesterone receptor composition and pathways for activation of receptors to the oncogenic refractory state in mammary tissue which is induced by high dose estrogen and progesterone treatment. The specific aims to accomplish the overall objectives are as follows. 1) To establish the quantitative complement of wild-type and variant estrogen and progesterone receptors in rat mammary epithelial and stromal tissue during mammary development, during MNU-induced tumorigenesis and after induction of a mammary state that is refractory to carcinogenesis by early treatment with estrogen and progesterone; 2) To establish the functional capacity of estrogen and progesterone receptor variants in vitro and in estrogen and progesterone receptor negative cell cultures.; 3) To determine whether the temporal and spatial expression of estrogen and progesterone receptor regulated target genes is age, hormone and growth factor regulated and to establish whether the refractory mammary phenotype that is induced by early administration of estrogen and progesterone is accompanied by alterations in the subsequent responses of these target genes to estrogen, or progesterone receptor can directly contribute to the oncogenic potential of rat mammary tissue. Together, these studies will provide valuable new insight into the biochemical mechanisms that underlie mammary carcinogenesis as well as th prevention of tumorigenesis by estrogen and progesterone.